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I have had trouble researching this. But are dual lands considered basic lands? I'm talking about the good ones like Taiga and Badlands. Back in the day, the lands only said....well, land. It wasn't untill later that they turned mountain, swamp, forest, ect. to 'Basic Land's.

Just wondering, because my friend says that they are basic lands, but all the new so-called duals are not basic.

Any response would help, thanks.

The Ravinca Duals are Duals, however i don't think the future sight ones are.
a good way of checking would be seeing the land type, the spot between the picture and ability.
the Dimir one was "Land - Swamp Island" so, anything that could search a Swamp or a Island could get it, same with the original ten.
the newer ones, i'm quite sure are not Duals though.

Dual lands are not basic, however they do count for spells that count for basic land types.

Example: You play Tribal Flames with a Plateau and a Bayou in play. It deals 4 damage, because Foundry counts for Mountain and Plains, while Tomb counts for Swamp and Forest, even though the lands themselves are not basic, they count for basic land types because they are "Land-Forest Swamp". The same is true for the Rav duals.

It's a little confusing at first, but eventually it makes sense.

The only basic lands are.

Plains
Island
Swamp
Mountain
Forest.

Also
The Snow-covered lands are also considered basic.
So you may fill you deck with either of the basic lands or their snow-covered alter egos. :]